In the semiconductor industry, one of the important procedures of advanced process control is control of geometrical dimensions of a structure being processed. Such measurements can be carried out by an AFM or CDSEM metrology tool. These are stand alone, bulky tools, the measurements being relatively slow. Another example for such kind of measurements is optical critical dimension metrology (Optical CD). In this technique, spectral and/or angular analysis of light reflected from a patterned (periodic) structure is used to measure the parameters that define/characterize the structure.
The Optical CD technique (known also as Scatterometry) is very efficient for measuring parameters of patterned (periodic) structures. Optical CD measurements are usually performed using a fitting procedure. According to this procedure, a theoretical model describing the structure under measurements is used to generate a theoretical signal; this signal is iteratively compared to the measured signal, while varying the model parameters or a previously prepared library of theoretical signals is used or a combination of both techniques is used. In cases where a parameter of the structure has no effect on the signal, this parameter is kept fixed in the model. When a parameter has a weak effect on the signal, it is also often kept fixed to prevent adding instability to the model. In the case of random or unknown parameters, they are usually not included in the model (un-modeled parameters).